1. FIELD OF THE INVENTION
The present invention pertains to strip-line transmission lines, which are generally used with printed circuit boards and the like to convey electrical signals from one point or component on a printed circuit board to another point or component on the printed circuit board. Strip-line transmission lines have many other uses but, since their use in conjunction with printed circuit boards is the most common, this use will be referred to throughout this disclosure.
2. DESCRIPTION OF THE PRIOR ART
There are two general types of transmission lines which are prior art to the present invention. The first general category is the micro strip transmission line which includes a single ground plane affixed to one surface of a dielectric material and a conductor affixed to the opposite surface of the dielectric material in spaced relation from the ground plane. This type of transmission line is unsatisfactory in many applications because it has very poor shielding and radiates or picks up extraneous energy easily. In fact, this transmission line is utilized as an antenna in many applications.
The second general category of transmission lines is the strip-line which includes two spaced apart ground planes affixed to opposite sides of dielectric material with a center conductor embedded in the dielectric material parallel with and spaced from each of the ground planes. This transmission line has relatively good shielding but the size becomes a serious manufacturing problem. For example, in a normal four-layer printed circuit board having a thickness of 0.062 inches, a center conductor spaced approximately 0.020 inches from one of the ground planes must have a width of approximately 0.013 inches to provide a characteristic impedance of 50 ohms. When the strip-line and the remainder of the printed circuit board are manufactured simultaneously the tolerance would typically be plus or minus 0.002 inches of a line or a little poorer than plus or minus 15%.